Why am I getting IP in CIDR errors when sending emails?
Summary
What email marketers say6Marketer opinions
Email marketer from Reddit explains that IP in CIDR errors could stem from either missing or incorrect reverse DNS records, or a negative IP reputation. The user recommends confirming that the sending IP has a valid PTR record pointing back to the server's hostname and that the IP is not blacklisted.
Email marketer from SparkPost suggests that IP reputation could be the cause, where a bad IP reputation is the reason the messages are getting blocked. Checking if the IP is on any blocklists.
Email marketer from StackOverflow explains it is necessary to ensure that your SMTP banner matches your hostname. Some receiving servers check if the hostname presented in the SMTP banner matches the PTR record of the connecting IP address and will reject emails if there's a mismatch.
Email marketer from Mailhardener explains that the most common reason for IP in CIDR errors is a missing or incorrect PTR record. The PTR record (reverse DNS) must match the hostname of your sending server. Mailhardener provides tools to check if your PTR record is correctly configured.
Email marketer from EmailDeliveryTips explains that it is important that your sending server's hostname resolves to the IP address. This will verify the forward and reverse DNS records are consistent. They also highlight the need to contact the ISP to fix the RDNS records.
Email marketer from DigitalOcean discusses DNS records. He explains that a missing or incorrect PTR record, which maps an IP address back to a domain name, is the most common reason. He suggests using online tools to check your reverse DNS configuration and contacting your hosting provider to set up the PTR record.
What the experts say4Expert opinions
Expert from Word to the Wise explains the importance of PTR records. A proper PTR record ensures reverse DNS is set up correctly, where the IP address resolves back to the domain name used for sending emails. They emphasize that mail servers often use this to verify the legitimacy of the sending server. They also explain about reverse DNS not being delegated.
Expert from Email Geeks explains that IP in CIDR likely refers to how your reverse DNS is set up.
Expert from Email Geeks shares potential causes, elaborating that the rule might be newly implemented or conditional. She also suggests checking at which point in the SMTP transaction the message is rejected (on connect, after EHLO, after RCPT TO, or after DATA) to identify differences between successful and unsuccessful deliveries and to investigate sender content and address collection processes.
Expert from SpamResource explains that using a dynamic IP address for sending email is a common cause for 'IP in CIDR' rejections. ISPs often block dynamic IPs to prevent spam because they are frequently used by home users who aren't expected to run mail servers. It also highlights the need for a static IP with a valid PTR record.
What the documentation says3Technical articles
Documentation from cPanel Documentation outlines how to configure reverse DNS for your server. It stresses the importance of contacting your ISP or hosting provider to set up the PTR record, as they control the reverse DNS zone for your IP address block. Also, the documentation highlights the importance of ensuring that the hostname resolves correctly to the IP address.
Documentation from MXToolbox describes that a missing or incorrect reverse DNS (PTR) record is the most likely cause of the error, as this is used to verify the sending server's identity. It is also explained that you can use MXToolbox to diagnose the DNS configuration.
Documentation from Microsoft Learn explains that an IP in CIDR error often relates to reverse DNS lookup failures. They recommend ensuring that your sending server's IP address has a valid PTR record that resolves back to the server's hostname. They also recommend verifying the DNS configuration for any discrepancies.